Halofluorocarbon (where halo means Cl, Br, I or F) based fluids have found widespread use in industry in a number of applications, including as refrigerants, aerosol propellants, blowing agents, heat transfer media, and gaseous dielectrics. For example, halo-fluoroalkanes, such as chlorofluoromethane and chlorofluoroethane derivatives, have gained wide spread use as refrigerants owing to there unique combination of chemical and physical properties. Similarly, because of their properties, halofluoro-carbon materials such as fluoro-trichloromethane (CFC-11) had become standard materials for aerosols and foam blowing agents.
Because of suspected environmental problems associated with the use of some of these agents, including relatively high global warming potentials (GWP) associated therewith, it is desirable to use agents having the lowest possible GWP in addition to low, preferably zero, ozone depletion potential (ODP). Thus there is considerable interest in developing such environmentally friendlier materials.
One solution to this problem involved substituting a hydrogen containing chlorofluoroalkane (HCFC) in place of the CFC. For example, dichlorotrifluoroethane, CHCl2CF3 (HCFC-123), dichlorofluoromethane (CHCl2F), and dichlorofluoroethane (CH3CCl2F) (HCFC-141b) were proposed as replacement agents. Over time, however, even these more environmentally acceptable materials were found to have shortcomings since they still contained chlorine and therefore still had an unacceptable ODP. Consequently, these materials have been targeted for eventual removal from use.
Recognizing the need to identify a non-chlorinated compound, hydrofluoro-carbon materials (HFCs) were identified as plausible replacements for the HCFCs. While the HFCs did not exhibit any substantial ODP values, they did have associated with them a global warming potential which presents a new set of issues for acceptance.
Because of the suspected environmental problems associated with the use of the various classes of compounds discussed above, it is desirable to continue to search for compounds having the lowest possible GWP and ODP. Thus there is considerable interest in developing environmentally friendlier materials for the applications mentioned above. One such class of compounds which meet the new needs are materials derived from a class of compounds referred to as fluoro-olefins and more specifically, fluorinated butenes.
However, while fluorinated butenes have zero ozone depletion values and very low global warming potential values, the toxicity, boiling point and other critical properties required to meet the applications specified above, can vary greatly from isomer to isomer. One particular isomer of fluorinated butenes which has the potential to fill many of the applications specified above is 1,1,1,4,4,4-hexafluoro-2-butene and more specifically, cis-1,1,1,4,4,4-hexafluoro-2-butene:

There are several methods for producing hexafluoro-2-butene, but such processes may give exclusively the trans-isomer. See, for example, the zinc reduction of 1,1,1,4,4,4-hexafluoro-2-iodobutene; K. Leedham and R. N. Hazeldine, J. Chem. Soc., 1954, 1634.
Processes that give a mixture of cis- and trans-isomers are likewise undesirable if a substantial proportion of the trans-isomer is formed. One reason is that the difference in boiling points for the two isomers is large (the trans-isomer boiling at about 9° C. and the cis-isomer boiling at about 32° C.). For applications that depend in large part on the boiling point of the fluorocarbon, the large difference in boiling points may mean that only one isomer is suitable and the other isomer therefore represents a yield loss. Another reason such a mixture is undesirable is that a good means for recycling the undesired trans-isomer is lacking. Ideally, a suitable process will provide the cis:trans isomers in a ratio of 10:1 or better.
Still other processes for cis-olefins suffer from co-production of the corresponding saturated alkane compound. In the present case, this means the co-production of 1,1,1,4,4,4-hexafluorobutane. This is likewise undesirable because it does not posses the low GWP that the corresponding butene does. Furthermore, like the trans-isomer, there is no convenient way to recycle this by-product.
One prior art method for making cis-1,1,1,4,4,4-hexafluorobutene (J. Am. Chem. Soc., 71, 1949, 298) involves reduction of hexafluoro-2-butyne with hydrogen (100 atmospheres) using Raney nickel catalyst at room temperature. Not only does this pressure require specialized equipment, but the conversion was only 82% and the product was a mixture of cis-hexafluoro-2-butene (41% yield) and hexafluorobutane (25% yield). Ideally the amount of over-reduced material should be less than 10%. Still more preferably, the total amount of trans-isomer and butane are together less than 10%.
R. N. Hazeldine, J. Chem. Soc., 1952, pp. 2504, also reported the reduction of hexafluorobutyne with Raney nickel at 60° C. and 15 atmospheres of hydrogen pressure to give cis-hexafluorobutene. Although some over-reduction to hexafluorobutane was mentioned, the yield of 91% is substantially better than the yield given in the reference cited above.
A few methods exist for the exclusive preparation of non-fluorinated cis-olefins to the exclusion of the corresponding trans-isomer. The most common of these is the catalytic reduction of alkynes. A number of catalysts may be employed for this transformation but they can, unfortunately, give a wide range of results and undesirable side reactions such as over-reduction to alkanes, formation of trans-olefins, and isomerization of cis to trans olefins. In addition, a wide range of variables can alter the results, such as temperature, mixing rate, solvent, and added reagents which may intentionally or unintentionally alter the reactivity of the catalyst.
For a general discussion of this chemistry see P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Chapter 4, Academic Press, 1967. For example, depending on the temperature, the reduction of acetylene dicarboxylic acid using Pd on BaSO4 can give either succinic acid (no double bond) at −18° C. or maleic acid (cis double bond) at 100° C., while the ratios of cis to trans products for the reduction of p-methoxyphenylacetylene carboxylic acid with the same catalyst were similar (20%±5% trans isomer) over a wide temperature range. See, S. Takei and M. Ono, Nippon Nogei Kagaku Kaisi 18 (1942b) 119.
Catalysts that have been used for the selective reduction of non-fluorinated alkynes to alkenes include Pd/C, Pd/BaSO4, Pd/BaCO3, and Pd/CaCO3. In order to achieve high selectivity, however, the use of quinoline as a catalyst modifier has been recommended whether the catalyst is Pd/C, Pd/BaSO4, or Lindlar catalyst, Pd/CaCO3/Pb. See, M. Hudlicky, Reductions in Organic Chemistry, 2nd Ed., ACS Monograph 188, 1996, p 8.
The Lindlar catalyst is often used for the reduction of hydrocarbon alkynes to cis-alkenes, modified further by the addition of an aromatic amine such as quinoline or pyridine. The amines, while often useful in improving reaction selectivity, are not desirable from the standpoint of their toxicity. The quality of the quinoline used may also affect the outcome. The Pd/CaCO3/Pb catalyst, modified with pyridine, was successfully used in the reduction of an alkyne bearing a single fluorine on the carbon adjacent to the triple bond to give the corresponding cis-alkene. See, M. Prakesch, D. Gree, and R. Gree, J. Org. Chem., 66 (2001) 3146.
As is well known in the art, however, fluorocarbons often behave quite differently compared to non-fluorinated alkanes, and perfluorinated compounds may behave quite differently than even partially fluorinated compounds of similar structure.